A detection of an interannual air-sea coupled signal along the subtropical front in the North Pacific

Interactions between the atmosphere and ocean are investigated for an interannual propagating signal in sea surface temperature (SST) anomalies in the North Pacific. The signal starts from the Philippine sea to the central North Pacific in 2-3 years with a dominant periodicity of about 5 years. The seasonally combined CEOF analysis for the high-pass filtered (8 Coherent propagating signature is found in the high-pass filtered latent heat flux, which generally has the largest amplitude than other heat flux components. The phase relation between the SST and latent heat flux indicate that larger latent heat fluxes are inputted into the ocean where the SST anomalies are warmer with slight phase lags of the SST to the latent heat flux. Propagating signatures are also prominent in sensible heat flux and in surface meridional wind speed. The propagating feature in the meridional wind speed is not observed higher than 850 hPa geopotential height, suggesting that the atmospheric propagating signals exist only in the surface boundary layer.

Amplitudes and phases of the interannual SST variation are estimated. The SST is assumed to be forced by the heat fluxes and is influenced by the Ekman transport. The estimated SST phases indicated a prominent propagation toward the east along the subtropical front, and agrees well to the observed SST phases. The estimated amplitude is qualitatively similar to the observed amplitude. The latent heat plays a dominant role in the western North Pacific, while in the central North Pacific the heat flux and Ekman transport contribute to the SST in a similar magnitudes. The latent heat anomalies should be related to the meridional advection of air-masses across the subtropical front. The SST gradient across the subtropical front also makes the amplitude of the Ekman advection significant.

These results indicate that the SST propagation is caused by the propagating atmospheric variations. In addition, the atmospheric propagating signature should reflect from the SST propagation, thought the detailed mechanism is not clarified. Therefore, the propagating signatures in the SST, latent heat flux, meridional wind and zonal wind are probably aspects of an air-sea coupled propagating mode, in which the subtropical front plays important roles.